Annealing air-stable magnetic materials having superior magnetic characteristics and method

ABSTRACT

A process for producing novel air-stable coated particles of a magnetic transition metal-rare earth alloy. An organometallic compound which decomposes at a temperature below 500*C is heated to produce a metal vapor which is contacted with particles of a transition metal-rare earth alloy to deposit a metal coating thereon. The coated particles are heated at a temperature ranging from 50*C to 200*C for a period of time sufficient to increase their intrinsic coercive force by at least 10 percent.

United States Patent 1191 Smeggil et al.

1451 Dec.24, 1974 ANNEALING AIR-STABLE MAGNETIC MATERIALS HAVINGSUPERIOR MAGNETIC CHARACTERISTICS AND METHOD [75] Inventors: John G.Smeggil, Elnora; Richard J.

Charles, Schenectady, both of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

22 F1166; June 22,1973

21 App1.No.:372,688

[52] US. Cl. 148/105, 75/.5 BA, 148/3157, 148/103, 117/107.2 [51] Int.Cl. H0lf l/02 [58] Field of Search 148/105, 31.57, 103,101, 148/613;75/.5 B, .5 BA; 117/100 M, 107.2 R, 127,, 130; 29/192 CP [56] ReferencesCited UNITED STATES PATENTS 2,933,415 4/1960 Homer et al. 117/100 M3,219,482 11/1965 Jenkin 148/63 3,342,587 9/1967 Goodrich et al. 117/100M 3,385,725 5/1968 Schmeckenbecher 117/130 3,479,219 11/1969 Haines eta1. 117/107.2 R 3,511,683 5/1970 Espenscheid et al 117/100 M 3,591,4287/1971 Buschow et a1 148/3157 3,615,914 10/1971 Becker 148/101 3,632,4011/1972 Sanlaville 117/100 M 3,684,593 8/1972 Benz et a1. 148/3157 OTHERPUBLICATIONS Heslop, R. et al., Organometallic Compounds, in InorganicChemistry, New York, 1967, p. 393 (QDl5lH47).

I-Iarwood, Applications of Organometaliic Compounds, New York, 1963, pp.339-343 (TP247A35).

Primary Examiner--Wa1ter R. Satterfield Attorney, Agent, or Firm-Jane M.Binkowski; Joseph T. Cohen; Jerome C. Squiilaro [57] ABSTRACT 3 Claims,N0 Drawings ANNEALING AIR-STABLE MAGNETIC MATERIALS HAVING SUPERIORMAGNETIC CHARACTERISTICS AND METHOD The present invention relatesgenerally to the art of making magnets. More particularly, it isconcerned with novel annealed metal-coated magnetic material powdershaving superior magnetic characteristics, and with magnets wherein theseannealed coated powders are the active magnetic components.

Magnetic properties of bulk magnetic materials hav ing largemagnetocrystalline anisotropies can be enhanced by reducing them topowders particularly those having an average particle size of less than10 microns. The asground powders can be incorporated in bonding media toprovide composite permanent magnets having properties substantiallysuperior to those of the bulk source materials. However, when goodmagnetic prop erties are attained in the as-ground powders, for example,cobalt-rare earth alloy powders, they tend not to be stable. As thepowders are exposed to air at room tem perature and at slightly elevatedtemperatures, their intrinsic coercive force, H which is a measure of amagnets resistance to demagnetization decreases irreversibly.Specifically, these magnetic powders are quite reactive'to oxygen andwater vapor in the atmosphere at room temperature, and they are evenmore so reactive at even slightly elevated temperatures, i.e., about100C, resulting in a significant loss in their intrinsic coercive force.Thus, a comparatively low value of intrinsic coercive force cansubstantially diminish the advantages to be gained by converting thebulk body to a powder, or producing the powder by some other technique,and fabricating a composite finished article from the powder.

The art has used sintering to produce magnets with substantially stableproperties from these powders. This process comprises compacting thepowder to form a green body and sintering the body at high temperatures,generally about l,0OC, in an inert atmosphere to produce a high densitycompact having a closed pore structure. Such a structure protects themagnet from the atmosphere resulting in long term stability of itsmagnetic properties. However, this method is expensive, since itrequires power-consuming equipment and handling procedures which aretime-consuming.

A more desirable approach to the fabrication of magnets using thesepowders, for example cobalt-rare earth alloy powders, would delete thesintering process and merely compact the aligned particles into thedesired shape with the aid of some kind of binder. However, to do thisrequires the use of air-stable, accordingly coated, cobalt-rare earthalloy particles.

Attempts to provide cobalt-rare earth alloy powder with a protectivemetal coating deposited from metal vapor of a molten metal have yieldedlimited success. For example, temperatures of 500C and highersignificantly deteriorate the magnetic properties of the loose powder.Such a method, therefore, can utilize only a very few low meltingmetals, which also must produce sufficient vapor pressures for effectivecoating deposition at temperatures not much higher than their meltingpoint, such as lead with a melting point of 328C or zinc with a meltingpoint of4l9C. However, most metals, especially those which are mostinert and generally the most desirable, have very high melting pointsand usually require temperatures significantly higher than their meltingpoints to produce vapor pressures which are effective for coating. Forexample, aluminum, a highly inert and desirable metal, melts at 660C andrequires significantly higher temperatures to produce vapor pressuresuseful for coating, and tungsten, another desirable metal, melts at3,370C. Not only do such high temperatures make deposition of the metalfrom the vapor of the molten metal impractical, but also these vaporswould be so hot as to significantly deteriorate the properties of thepresent magnetic transition metal-rare earth alloy powders.

Similarly, the coating of cobalt-rare earth particles by electrolessplating techniques is not highly attractive since these methods requireplacing the very fine, generally 10 micron average particle size, andconsequently very reactive cobalt-rare earth powders into contact withan aqueous solution which is highly acidic and results in thedissolution of significant amounts of material. These plating techniquesalso do not appear to produce a uniform coating on these fine particles.In addition, long term deleterious effects on the magnetic properties ofthe cobalt-rare earth powders can be expected from the direct effects ofthe acidic aqueous solutions or from amounts of water entrapped withinthe metal coating in the thin layer of Co and Sm O surrounding eachparticle which reacts slowly with the base cobalt-rare earth alloy.

In copending US. Pat. application Ser. No. 372,691 filed of even dateherewith in the names of Richard J. Charles and J ohn G. Smeggilentitled Air-Stable Magnetic Materials And Method, which is incorporatedherein by reference, there is disclosed a process which overcomes thedisadvantages of the prior art. It provides a solution to the oxidationproblem of these reactive materials and obviates the sintering procedureby coating the powders with a coherent and non-reactive metal withoutsignificantly affecting the magnetic properties of the powder. Brieflystated, the process disclosed in that copending application comprisesproviding particles of a magnetic transition metal-rare earth alloy,heating an organometallic compound to decompose said compound andproduce a metal vapor, and contacting said metal vapor with saidparticles to deposit a coating of metal thereon. These metal-coatedalloy particles can be magnetized and pressed into a compact to form amagnet. They can also be magnetized before or after distribution in. anon-magnetic matrix and the resulting mixture pressed into a compactuseful as a magnet. In the present invention, these metal-coatedparticles are annealed in air to increase their intrinsic coercive forceby at least 10 percent.

Briefly stated, the process of the present invention comprises providingparticles of a magnetic transition metal-rare earth alloy, heating anorganometallic compound to decompose said compound and produce a metalvapor, contacting said metal vapor with said particles to deposit acoating of metal thereon and heating said coated alloy particles at atemperature ranging from about to 200C for a period of time sufficientto increase their intrinsic coercive force by at least l0 percent.

In the present process a magnetic transition metalrare earth alloy,e.g., TRE, where '11 is a transition metal and RE is a rare earth metal,is used in particle form. The transition metal is selected from thegroup consisting of cobalt, iron, nickel, manganese and alloys thereof.

The rare earth metals useful in the present process are the elements ofthe lanthanideseries having atomic numbers 57 to 71 inclusive. Theelement yttrium (atomic number 39) is commonly included in this group ofmetals and, in this specification, is considered a rare earth metal. Aplurality of rare earth metals can also be used to form the presentintermetallic compounds which, for example may be ternary, quartenary orwhich may contain an even greater number of rare earth metals asdesired. Mischmetal, an abundant common alloy of rare earth metals, isparticularly advantageous.

Representative of the cobalt-rare earth compounds useful in the presentinvention are cobalt-cerium. cobalt-praseodymium, cobalt-neodymium,cobaltpromethium, cobalt-samarium, cobalt-europium, cobalt-gadolinium,cobalt-erbium, cobalt-thulium, cobalt-ytterbium, cobalt-lutecium,cobalt-yttrium, cobaltlanthanum and cobalt-mischmetal. Examples ofspecific ternary compounds include cobalt-ceriumpraseodymium-mischmetal.

'Transition metal-rare earth intermetallic alloys or compounds exist ina variety of phases and each phase may vary in composition. A materialsubstantially comprised of the T RE single phase is particularlypreferred in the present invention since this phase has shown the mostdesirable combination of magnetic properties.

The transition metal-rare earth compound or alloy of the present processcan be prepared by a number of methods. For example, it can be preparedby melting the transition metal and rare earth metal together in theproper amounts under a substantially inert atmosphere such as argon andallowing the melt to solidify.

The alloy can be converted to particulate form in a conventional manner.For example, it can be crushed to a coarse size and then pulverized to afiner form by, for example, fluid energy milling in a substantiallyinert atmosphere. Alternatively, the powder can be produced initially bya reduction-diffusion process as set forth in copending application Ser.No. l72,290, now US. Pat, No. 3,748,193, filed on Aug. 16, 1971 in thename of Robert E. Cech. Also, in some instances, it may be desirable togrind sintered compacts of these powders to a desired particle size.

The particle size of the transition metal-rare earth alloy used in thepresent process may vary. It can be in as finely divided a form asdesired. For best magnetic properties, average particle size will rangefrom about 1 micron or less to about 10 microns. Larger sized particlescan be used, but as the particle size is increased, the maximum coerciveforce obtainable is lower because the coercive force decreases withincreasing particle size.

In the present process, an organometallic compound is used which can bea solid, liquid or gas at room temperature and which decomposes attemperatures lower than 500C to produce a metal vapor. The metal vaporis contacted with the present TRE alloy powder to deposit metal thereonto form a continuous coating of metal which protects the powder from theatmosphere. The present coating process yields a coherent substantiallyuniform layer of metal and is accomplished in an atmosphere in which thereactants are inert. Typical inert atmospheres which are suitable forthe coating process include argon, nitrogen or a vacuum. No water vaporor oxygen gas is present to degrade the magnetic properties of the alloymaterials.

In carrying out the present process, the organometallic compound and theTRE alloy powder are preferably admixed to produce a substantiallyintimate mixture so that when the organometallic is decomposed, theresulting metal vapor, which deposits metal on contact with the surfaceof the alloy powder, will be distributed substantially uniformlythroughout the powder to effectively deposit a continuous coatingthereon and thereby provide a barrier to the atmosphere. If desired,mixing can be continued during decomposition of the organometallic tomaintain a substantially intimate mixture. When the organometalliccompound is a solid at room temperature, it is preferably used in a finepowder form in order that it can form an intimate mixture with the TREallow powder. When the organometallic is a liquid at room temperature,it should be admixed with the alloy powder to thoroughly wet thesurfaces thereof. Alternatively, the organometallic compound may bevaporized and flowed through the alloy particles in such form. In yetanother technique, the organometallic can be decomposed and theresulting metal vapor carried by an inert gas, such as argon, intocontact with the alloy powder to deposit a coating thereon.

In some instances the uniform deposition of a metallic coating on TREalloy powder may be hindered because the organometallic is notferromagnetic in nature while the TRE alloy powder is. This problem mayparticularly occur when the organometallic compound is a solid at roomtemperature and when the organometallic and TRE allow powder are simplymixed together prior to and/or during the coating process itself. Due totheir magnetic nature, the TRE alloy powders may tend to conglomerateand so separate from the organometallic compound resulting in thedeposition of an uneven metallic coating. The solution to this problemis to use a liquid organic carrier compound which will not attack and somagnetically degrade the TRE alloy powders and which will dissolve atleast to a minor extent the organometallic desired for the coatingoperation. The organic carrier liquid can then be used to eitherdissolve or to form a slurry with the desired organometallic. Then thissolution or slurry can be admixed with the TRE alloy powders in aconventional manner, e.g., either mechanically or magnetically agitated,to produce a substantially intimate mixture. The organic carrier liquidcan then be removed by either gentle heating or the application of avacuum or a combination of both of these. The organometallic willprecipitate from the solution or slurry and coat the TRE alloy particlessubstantially uniformly. The TRE alloy particles now coated with anorganometallic powder can then be heated under the appropriateconditions to decompose the organometallic and leave the desiredmetallic coating. Representative of the organic liquid carriers usefulin the present invention are carbon tetrachloride; 1,1 ,ltrichlorotrichloroethylene; 1,1,1 trichloroethane and dimethylsulfoxide.

A number of conventional techniques can be used to carry out the presentprocess. However, best results are obtained by the use of a fluid bedreactor supporting the transition metal-rare earth alloy powder acrosswhich flows an inert gas stream bearing a significant partial pressureof the organometallic compound to be decomposed. Around-the fluid bed isa furnace supplying a sufficient amount of heat to decompose theorganometallic compound in the gas phase to produce a metal vapor whichdeposits a metal coating on the powder particles. The partial pressureof the organometallic compound should be sufficient to yield, whendecomposed, a partial pressure of metal vapor sufficient to effectivelydeposit a metal coating on the particles which substantially envelopsthe particles in a reasonable period of time, i.e., less than 8 hours.The particular useful partial pressure of organometallic compound isdeterminable empirically and generally is at least about 10' atmosphere.

Alternatively, another coating technique makes use of the magneticproperties of the particles themselves to help produce continuouscoatings. For example, a mixture of the transition metal-rare earthalloy powder and the desired organometallic is stirred in a nonmagneticcontainer by an external magnet while the temperature is raised to thepoint at which the organometallic decomposes producing a metal vaporwhich deposits metal on contact with the particles. In this embodimentan inert gas stream is passed through the container or a vacuum ispulled on the container during processing.

In the present process, the amount of organometallic compound used isdeterminable empirically. It should be used in an amount which, ondecomposition, is sufficient to produce an amount of metal vapor whichcondenses on the exposed surfaces of the alloy particles to form acontinuous coating of metal thereby preventing penetration by theatmosphere. Specifically, the amount of organometallic compound usedshould, upon decomposition, yield a significant partial pressure ofmetal vapor, generally at least about 10" atmosphere, sufficient toeffectively coat the exposed surfaces of the alloy particles with acontinuous coating of metal. The organometallic compound may decomposeinitially to yield the metal vapor or it may decompose to yield anotherorganometallic vapor which is then decomposed to give the metal vapor.Preferably, the organometallic compound should be used in an amountwhich on decomposition, produces the metal in an amount ranging from 1to 5 percent by weight of the alloy powder. From formulas and atomicweights, the weight relationships between the substances in the reactioncan be calculated readily. Amounts of deposited metal less than 1percent by weight of the alloy powder are likely to result in adiscontinuous coating whereas amounts of deposited metal significantlygreater than 5 percent by weight of the alloy powder will dilute themagnetic properties of the powder. Best Results are attained with themetal being deposited in an amount of 2 percent by weight of the alloypowder.

The minimum thickness of the metal coating need only be sufficient tomake it continuous, e.g., at least a film-forming thickness which isabout 1 microinch, to prevent air from penetrating to the surface of thealloy particles. In some instances where a metal may form a porousoxide, thicker continuous coatings of the metal should be deposited tomake the outer portion of such a metal coating available to be oxidizedby the air leaving an inner continuous metal coating to maintain thestability of the magnetic properties of the alloy particles. However, anumber of metals, for example, aluminum, form non-porous oxides whichare effective barriers to air. Metal coatings significantly thicker thanthat necessary to provide the alloy particles with an effective barrierto the atmosphere provide no particular ad vantage since they do notimprove magnetic stability and prevent a close packing of the: alloyparticles in the non-magnetic matrix thereby diluting the magneticproperties. Metal coatings thicker than necessary can be useful if suchmetal is also to serve as a matrix or partial matrix for supporting theparticles.

In the present invention the metal coating should have a number ofproperties. Specifically. it should provide a barrier to the atmosphere,and also. if desired. the metal coating can be chosen for some otherdesired property, e.g., ductility. The metal, itself. should have nosignificant deteriorating effect on the magnetic properties of thepowder. It should be non-magnetic or so weakly magnetic as not todiminish the magnetic properties of the powder significantly.

In the present invention, the particular deposited coating can becomposed of more than one metal to form an alloy depending on theparticular properties desired. A plurality of metals, for example Cu andZn, can either be deposited sequentially or concommitantly in anyproportion to form an alloy coating on the particles.

The non-metallic products of decomposition are gaseous, or will usuallybe evaporated from the alloy particles during the decomposition step, orcan be evaporated therefrom at tempertures below 500C, such removalbeing preferably promoted by a flowing atmosphere or a substantialvacuum.

In the present process, there are a number of useful organometalliccompounds which decompose at temperatures below 500C. Typical of theseis triisobutylaluminum as a source of aluminum. Specifically, the metalcoating of aluminum can be deposited according to the followingreactions:

This operation must be accomplished at reduced pressures because thetriisobutylalumlinum cant be successfully distilled above 10mm Hg.

There are a number of advantages to the use of triisobutylaluminum andother organo'metallics which decompose in a similar manner. Oneadvantage is that the low temperature at which this organometallicdecomposes will not affect the magnetic properties of the transitionmetal-rare earth alloy powder. Another advantage is that the amount ofchemical interaction be tween the aluminum and the alloy powder shouldbe minimal at these temperatures. Yet another advantage is that thehydrogen gas present can be expected to reduce any surface oxidespresent on the alloy particles. Also, although it has been reported thatcobalt-rare earth alloy powder absorbs hydrogen, pressures of H somewhatin excess of 76cm Hg are needed. Therefore this proposed process workingwith a low residual pressure, -lOmm Hg, should minimize deleteriouseffects due to hydrogen absorption. In addition, the organometallicdecomposition reaction is relatively clean and yields products, exceptfor elemental Al, which are gases and are accordingly easily removedfrom the coated powders.

A typical example of an organometallic useful in the present inventionfor the vapor deposition of copper is phenylcopper C H Cu, whichthermally degrades according to the following reaction:

This reaction affords advantages similar to those listed for the Aldeposition, and in addition, it takes place at a very low temperature.

Table 1 lists a partial series of elemental metals useful as coatingsand their organometallic compounds suitable for the present process. inthe present invention, metal carbonyls are assumed to be organometalliccompounds.

TABLE I Metal Organometallic Compound Cu Copper formate, Cu(CH O Copperacetylacetonate [Cu(CH COCHCOCHM] 4 Methylcopper, CuCH Ni Nickelcarbonyl. Ni(CO) Fe lron carbonyl Fe(CO) 'Cr Chromium carbonyl. Cr(CO)Bisbenzene chromium. Cr(C.,H.,) Mo Molybdenum carbonyl. Mo(CO)Bisbenzene molybdenum. Mo(C H Benzene molybdenum carbonyl. C H Mo(CO) WDibenzene tungsten, W (C m) Mesitylenc tungsten carbonyl. (CH C H.W(CO);, Tungsten carbonyl. W(CO) Ru Ruthenium carbonyl, Ru(CO),. and/orRu (CO),. lr Iridium carbonyl, lrflCO), V Vanadium carbonyl. VtCO)Bisbenzene vanadium. (C,,-H,,),V Hf Dicyclopentadienyl hafniumdichloride, (C,,H l-lf 2 Ta Tantalum methylcyclopentadienyltetracarbonyl,

CH C H TMCO). Nb Niobium methylcyclopendadienyl tetracarbonyl.

CH C5H Nb(CO)4 Zn Diethylzinc, Zn(C H Dimethyl zinc. Zn(CH )2 Zincacetylacetonate [Zn(CH COCHCOCH Be Diethyl beryllium, (C HQ Be MgDiphenylmagnesium. Mg(C l-l Diethylmagnesium. Mg(C H Sn Tetramethyl tin.Sn(CH Bi Trimethyl bismuth. Bi(CH Au Diethyl gold bromide. [(C H AuBi1Pb Tetraethyl lead. Pb(C H Mn Dicyclopentadienyl manganese. (C H Mn ReRhenium carbonyl. Re CO) Rh Rhodium carbonyl, Rh (CO),, TiDicyclopentadienyl titanium. (C H Ti In addition to the aboveorganometallics listed in Table I, there are a number oftrifluoroacetylacetonates and hexafluoroacetylacetonates of variousmetals, e.g., Zn and Zr which could yield the desired metal coating.

The present solid metal-coated particles are annealed to increase theirintrinsic coercive force by at least 10 percent. To carry out theannealing, the coated particles are heated at a temperature ranging fromabout 50 to about 200C. Specifically, the annealing temperature shouldnot be so high as to deteriorate the barrier coating of the particlessignificantly. On the other hand, temperatures below 50C are noteffective. Annealing can be carried out in an atmosphere in which thecoated particles are inert, for example, argon, or in a substantialvacuum or in air. The particular annealing period of time to increaseintrinsic coercive force by at least 10 percent depends largely onannealing temperature and can range from 30 minutes to 100 hours withthe longer times being required at lower temperatures.

The present annealed coated particles are useful in the manufacture ofmagnets which are air-stable. e.g.. their magnetic properties do notdeteriorate significantly in air, at room temperature as well as atelevated temperatures that do not affect the barrier coatingsignificantly. Specifically, the annealed coated alloy particles of thepresent invention can be incorporated in a non-magnetic matrix to formmagnets. The annealed coated particles can be magnetized before or afterincorporation in the non-magnetic matrix. as desired. to produce themagnet.

The non-magnetic matrix used in forming the magnets of the presentinvention can vary widely. It can be. for example, a plastic or resin,an elastomer, or rubber, ora non-magnetic metal such as, for example,lead, tin, zinc, copper or aluminum. The extent to which the coatedalloy particles are packed in the matrix depends largely upon theparticular magnet properties desired.

Magnets having useful magnetic properties for a wide range ofapplications are produced when the annealed coated alloy particles ofthe present invention are incorporated in a non-magnetic matrix andmagnetized. The magnets of the present invention are useful intelephones. electric clocks, radios, television, and phonographs. Theyare also useful in portable appliances, such as electric toothbrushesand electric knives, and to operate automobile accessories. lnindustrial equipment, the present magnets can be used in such diverseapplications as meters and instruments, magnetic separators, computersand microwave devices.

All parts and percentages used herein are by weight unless otherwisenoted.

The invention is further illustrated by the following examples.

In the following examples the intrinsic coercive force of each samplewas measured at room temperature. Specifically, a specimen of the powderwas prepared for magnetic measurement by introducing it into a body ofmolten paraffin wax in a small glass tube and cooling the wax in analigning magnetic field of 20,000 oersteds to align the particles alongtheir easy axis until the paraffin solidified. The intrinsic coerciveforce of the sample was then measured after applying a magnetizing fieldof 30,000 oersteds.

EXAMPLE 1 A sintered body of compacted CoSm alloy powder, preparedsubstantially as set forth in US. Pat. No. 3,655,464, was ground to apowder using a jaw crusher and a jet mill. The alloy powder wascomprised substantially of Co Sm phase and a minor amount of Co Smphase. Approximately 5 grams of the alloy powder having a size of 325mesh (US. Standard Screen Size), e.g., an average particle size of about6 microns, were placed in a U-tube along with about 1 gram of copperacetylacetonate, present as the hydrate Cu(CH COCHCOCH .2H- O, and inpowder form which was calculated to yield copper in an amount of about2% by weight of the alloy powder. A slow stream of argon gas was passedthrough the U-tube across the mixture which was mixed by moving a magnetbeneath it. After a few minutes, a period of time considered suitablefor the removal of the majority of oxygen present by the argon stream,the mixture was first gently heated with Meeker burner and occasionallyduring the entire heating process a magnet was used to stir p themixture to assist both in driving off the water of hydration and inobtaining a more uniform metal coating on the alloy particles. Thetemperature of the mixture was then raised slowly to about 400C toaccelerate the decomposition of the copper acetylacetonate.

After a minute or two a brown coating was observed being deposited onthe alloy particles and on the walls of the U-tube. Heating wascontinued for several more minutes then stopped. The coated alloyparticles were allowed to cool to room temperature with argon continuingto flow over them. After the powders were cool a magnet was used toseparate the coated alloy particles from a small amount of non-magneticmaterial present, presumably unreacted copper acetylacetonate andelemental Cu metal.

The alloy particles were examined under an optical microscope and byscanning electron microcopy. They appeared to be enveloped by acontinuous uniform brown coating of copper metal.

The intrinsic coercive force, H ofa portion of these copper-coatedparticles, as well as of a portion of the uncoated particles of the samecomposition and size, was determined and the results are shown in RunNo. I of Table II.

The copper-coated particles, as well as the uncoated particles of thesame composition and size, were placed in an oven having an airatmosphere and maintained at a temperature of 92C. At the end of eachperiod of time indicated in Table II, a portion of the copper coatedparticles as well as a portion of the uncoated particles were removedfrom the oven and cooled in air to room temperature. The intrinsiccoercive force, H of each such portion was then determined and theresults are shown in Table II.

As shown by Table II, the copper-coated particles had an intrinsiccoercive force initially higher than that of the uncoated particles, andafter being heated at 92C in air for periods of time as long as 112hours, the intrinsic coercive force of the copper-coated particlesimproved significantly whereas that of the uncoated particlesdeteriorated significantly. This indicates that the copper coating wascontinuous and provided an effective barrier to the atmosphere toprevent atmospheric oxygen from reacting with and degrading the magneticproperties of the Co Sm powder. In addition, the significant increase inintrinsic coercive force of the copper-coated particles after heatingmay be due to re moval of sites for reverse domain initiation on the nowprotected surface of the alloy particles.

EXAMPLE 2 In this example the cobalt-samarium alloy powder used wassubstantially the same as that set forth in Example l. The intrinsiccoercive force of portions of the powder was determined before and afterannealing in an air oven at 150C for 30 minutes and the results areshown in Run No. 6 of Table III.

Portions of the alloy powder were coated with metal substantially as setforth in Example 1. Specifically, chromium hexacarbonyl, Cr(CO) in anamount of 4 percent by weight of the alloy powder, was admixed therewithand the mixture placed in a U-tube where it was maintained under astream of argon and substantially continuously mixed with a magnet. Itwas calculated that in this mixture chromium hexacarbonyl would yieldchromium in an amount of 1 percent by weight of the alloy powder. At atemperature of 400C the chromium hexacarbonyl decomposed, and afterabout a minute, a silvery coating was observed on the alloy powder andon the walls of the tube. Heating and mixing was continued for about 10additional minutes to insure complete coating of the particles withchromium and then stopped. The chromium-coated alloy particles werecooled to room temperature under argon. A magnet was then used toseparate the coated particles from a small amount of non-magneticmaterial present. The intrinsic coercive force of a portion of thecoated powder was determined and is shown in Run No. 7. The remainingcoated powder was heated in an air oven at C for 30 minutes, then cooledto room temperature and its intrinsic coercive force determined as shownin Run No. 7 of Table III.

The procedure used in Run No. 8 was the same as that of Run No. 7 exceptthat chromium hexacarbonyl was used in an amount of 20 percent by weightof the alloy powder which was calculated to yield chromium in an amountof 5 percent by weight of the alloy powder.

In Run No. 9 triisobutylaluminum was used in an amount which coated thealloy particles with aluminum in an amount of about 2 percent by weightof the alloy powder. The coating procedure differed from Run No. 7 inthat a double U-tube was used wherein the triisobutylaluminum was placedin one U-curve and the alloy powder was placed in the second U-curve.The triiso butylaluminum was decomposed at a temperature of 250C and theresulting aluminum vapor was carried by the argon stream into contactwith the alloy powder where it condensed on the powder which was beingcontinuously mixed with a magnet to insure the deposition of acontinuous uniform coating of aluminum.

The metal-coated particles-of Runs 7-9 were examined under an opticalmicroscope and a scanning elec tron microscope. They appeared to becompletely and substantially uniformly coated by metal.

The intrinsic coercive forces of the uncoated and coated powders, beforeand after annealing in air, are given in Table III.

As illustrated by Table III, the intrinsic coercive force of Run No. 6,the uncoated control powder, deterioilll rated significantly after 30minutes at 150C whereas the intrinsic coercive force of the metal coatedparticles of Run Nos. 7 through 9, which illustrate the presentinvention, increased significantly after the annealing treatment in air.This indicates that the present metal-coated particles provided aneffective barrier to the atmosphere to prevent atmospheric oxygen andmoisture from reacting with and deteriorating the magnetic properties ofthe present alloy powder.

In copending US. Pat. application Ser. No. 372,690 entitled FabricationOf Matrix Bonded Transition Metal-Rare Earth AlloyMagnets filed of evendate herewith in the names of Richard J. Charles and John G, Smeggilthere is disclosed a process for producing an air-stable porous magneticcompact which comprises admixing particles of a transition metal-rareearth alloy with an organometallic compound which decomposes at atemperature below 500C and pressing the misture to form a green body.The green body is heated to decompose the organometallic compound toproduce a non-metallic product and a metal vapor. The metal vapordeposits an interconnecting continuous coating of metal on the exposedsurfaces of the pressed alloy particles thereby preventing penetrationby the atmosphere, and the non-metallic product is outgassed from thebody leaving the resulting coated compact porous.

The above cited application is, by reference, made part of thedisclosure of the present application.

What is claimed is:

l. A process for increasing the intrinsic coercive force of magneticallyair-stable cobalt-rare earth alloy particles coated with a metal havinga melting point above 500C which comprises annealing the coatedparticles in an inert atmosphere or in a substantial vacuum orin air ata temperature ranging from about 50 to 200C for a period of time rangingfrom 30 minutes to hours, said annealing increasing the intrinsiccoercive force of said coated cobalt-rare earth alloy particles by atleast 10 percent, said magnetically air-stable particles having beenproduced by providing particles of cobalt-rare earth alloy having anaverage size up to about 10 microns, providing an organometalliccompound which at a temperature below 500C decomposes and yieldsproducts of decomposition consisting of gaseous non-metallic product anda metal vapor. placing said compound and said particles in asubstantially inert atmosphere which is a flowing atmosphere or asubstantial vacuum, heating said organometallic compound at atemperature below 500C and substantially completely decomposing it andproducing said gaseous product of decomposition and a metal vapor,contacting the resulting metal vapor with the cobaltrare earth alloyparticles depositing a coherent substantially uniform metal coatingwhich at least envelops the particles providing an effective barrier tothe atmosphere and which has no significant deteriorating effect ontheir magnetic properties and diffusing away the non-metallic gaseousproduct of decomposition, said organometallic compound being used in anamount which on decomposition yields a partial pressure of metal vaporof at least about 10 atmosphere and produces the metal in an amountranging from 1 percent to 5 percent by weight of said cobalt-rare earthalloy particles and said deposited metal having a melting point above500C.

2. A process according to claim 1 wherein said alloy is acobalt-Samarium alloy.

3. A process according to claim 1 wherein said organometallic compoundis copper acetylacetonate.

1. A PROCESS FOR INCREASING THE INTRINISIC COERCIVE FORCE OFMAGNETICALLY AIR-STABLE COBALT-RARE EARTH ALLOP PARTICLES COATED WITH AMETAL HAVING A MELTING POINT ABOVE 500*C WHICH COMPRISES ANNEALING THECOATED PARTICLES IN AN INERT ATMOSPHERE OR IN A SUBSTANTIAL VACUUM OR INA TEMPERATURE RANGING FROM ABOUT 50* TO 200*C FOR A PERIOD OF TIMERANGING FROM 30 MINUTES TO 100 HOURS, SAID ANNEALING INCREASING THEINTRINSIC COERCIVE FORCE OF SAID COATED COBALT-RARE EARTH ALLOYPARTICLES BY AT LEAST 10 PERCENT, SAID MAGNETICALLY AIR-STABLE PARTICLESHAVING BEEN PRODUCED BY PROVIDING PARTICLES OF COBALT-RARE EARTH ALLOYHAVING AN AVERAGE SIZE UP TO ABOUT 10 MICRONS, PROVVIDING ANORGANOMETALLIC COMPOUND WHICH AT A TEMPERATURE BELOW 500*C DECOMPOSESAND YIELDS PRODUCTS OF DECOMPOSITION CONSISTING OF GASEOUS NON-METALLICPRODUCT AND A METAL VAPOR, PLACING SAID COMPOUND AND SAID PARTICLES IN ASUBSTANTIALLY INERT ATMOSPHERE WHICH IS A FLOWING ATMOSPHERE OR ASUBSTANTIAL VACUUM, HEATING SAID ORGANOMETALLIC COMPOUND AT ATEMPERATURE BELOW 500*C AND SUBSTANTIALLY COMPLETELY DECOMPOSITING ITAND PRODUCING SAID GASEOUS PRODUCT OF DECOMPOSITION AND A METAL VAPOR,CONTACTING THE RESULTING METAL VAPOR WITH THE COBALT-RARE EARTH ALLOYPARTICLES DEPOSITING A COHERENT SUBSTANTIALLY UNIFORM METAL COATINGWHICH AT LEAST ENVELOPS THE PARTICLES PROVIDING AN EFFECTIVE BARRIER TOTHE ATMOSPHERE AND WHICH HAS NO SIGNIFICANT DETERIORATING EFFECT ONTHEIR MAGNETIC PROPERTIES AND DIFFUSING AWAY THE NON-METALLIC GASEOUSPRODUCT OF DECOMPOSITION, SAID ORGANOMETALLIC COMPOUND BEING USED IN ANAMOUNT WHICH ON DECOMPOSITION YIELDS A PARTIAL PRESSURE OF METAL VAPOROF AT LEAST ABOUT 10-7 ATMOSPHERE AND PRODUCES THE METAL IN AN AMOUNTRANGING FROM 1 PERCENT TO 5 PERCENT BY WEIGHT OF SAID COBALTRARE EARTHALLOY PARTICLES AND SAID DEPOSITED METAL HAVING A MELTING POINT ABOVE500*C.
 2. A process according to claim 1 wherein said alloy is acobalt-samarium alloy.
 3. A process according to claim 1 wherein saidorganometallic compound is copper acetylacetonate.